Polytetrafluoroethenes (PTFE's, also often referred to as polytetrafluoroethylenes) have found wide application due to their high chemical inertness, low friction properties, non-stick properties, high melting points and thus high service temperatures and thermal stability. These properties have made PTFE's a well known material and the material of choice for making protective coatings, sealing materials like valves, washers and O-rings, implants, insulators, membranes, films, gaskets for use in household applications, architecture, and medical, chemical, aircraft and automotive industry.
According to international industry standards tetrafluoroethene (TFE) homopolymers and copolymers with up to 1% by weight of other perfluorinated monomers can be called PTFE (see, for example, DIN EN ISO 12086-1). Furthermore, the polymers must have melting point within the range of 327+/−10° C. to qualify as PTFE.
To make shaped articles from PTFE, the polymers need to have sufficient mechanical properties, such as a tensile strength of at least 10 MPa and an elongation at break of at least 20%. Otherwise, the materials are too brittle to be shaped into articles. To have such mechanical properties the PTFE polymers must have a sufficiently high molecular weight, typically about 106 g/mole or greater. However, PTFE polymers at such high molecular weight also have a very high melt viscosity (about 1010-1013 Pa·s at 380° C.). This results in PTFE having a melt flow index (MFI) of less than 0.1 g/10 min at 372° C. using a 5 kg load (MFI 372/5 of 0 g/10 min).
The MFI measures the amount of polymer that can be pushed through a die at a specified temperature (here 372° C.) using a specified weight (here 5 kg). Thus, the MFI is a measure for the suitability for melt-processing of a polymer. Fluoropolymers with an MFI (372/5) of less than 0.1 g/10 min are considered not melt-processable. They cannot be processed from the melt by ordinary melt-processing techniques like, for example, melt extrusion or injection molding to make shaped articles.
Therefore, to make shaped articles from PTFE special processing techniques have to be used, like ram extrusion or cold compression molding and sintering. Typically, PTFE's are processed by preparing blocks, which are then sintered to fuse the polymer particles. The sintered billets are then skived or machined into shaped articles. These techniques may lead to inhomogeneous products containing cavities as a result of imperfect fusion of the PTFE particles during sintering. Furthermore, machining and skiving methods are economically inefficient because they produce considerable amounts PTFE waste.
Therefore, attempts have been made to prepare fluoropolymers that are melt processable.
One attempt is the production of low molecular weight polytetrafluoroethene, so-called “PTFE micropowders”. PTFE micropowders have a high melting point like high molecular PTFEs but have a much lower melt viscosity (typically far below 105 Pa·s. at 382° C.). They have an MFI (372/5) of greater 0.1 g/10 mins and theoretically could be processed from the melt. However, PTFE micropowders are brittle and do not have mechanical properties suitable for making shaped articles. Micropowder material formed by melt processing breaks upon cooling or cannot be released from the mold without breaking PTFE micropowders thus do not have any mechanical properties suitable for making shaped PTFE articles. Therefore, PTFE micropowders are used as solid lubricants or as additives to impart low friction or low energy surface properties to other polymers. PTFE micropwoders are commercially available under the trade designation “Dyneon TF 9201” or “Dyneon TF 9207” from Dyneon LLC, Oakdale, Minn., USA or under the trade designation Zonyl (e.g. Zonyl 1000, Zonyl 1100, Zonyl 1200, Zonyl 1400, and Zonyl 1500) from DuPont de Nemours, Wilmington, Del., US.
Another attempt of providing melt-processable fluoropolymers involves copolymerising comparatively high amounts of perfluoro alkyl vinyl ethers (PAVE's) with TFE. Typical amounts of copolymers range from 1 to 5 mol % (which corresponds to 1.7% to 8.4% wt % in case of perfluoro methyl vinyl ether (PMVE)—the smallest of the perfluoro alkyl vinyl ethers). These types of fluoropolymers are referred to in the art as “PFA's”. PFA's have a molecular weight of 1 to 5×105 g/mol and a melting point between 300 and 315° C. (compare Modern Fluoropolymers, John Schiers, Wiley & Sons New York, 1998, pp 223 to 232). PFA's are melt-processable with sufficient mechanical properties to make shaped articles. However, due to their lower melting point they have a lower service temperature and thermal stability than PTFE's.
In U.S. Pat. No. 6,531,559 and U.S. Pat. No. 7,160,623 to Smith et al. another attempt to provide melt-processable PTFE's which melting points greater than 320° C. has been described. According to these documents such polymers may be prepared by blending various PTFE grades of different molecular weight and MFI's. While some of those blends are reported to be melt-proccessable and are reported to be releasable from molds without breaking these materials tend to be inhomogeneous and may thus be disadvantageous. U.S. Pat. Nos. 6,531,559 and 7,160,623 also appear to suggest making a TFE-comonomer with hexafluoropropene (HFP) or a perfluoro alkyl vinyl ether (PAVE) in amounts of less than 3 or less than 0.5 mole % for producing a melt-processable PTFE. However, while these documents provide examples for PTFE blends they do not provide any description of how such melt-processable copolymers can be prepared nor do they provide any examples of such copolymers. Indeed, when using HFP or a PAVE as comonomers with TFE in amounts as low as 1.0% wt. the resulting polymers with melting points greater than 315° C. were found to be so brittle that mechanical properties like tensile strength or elongation at break could not even be measured (see comparative examples herein below). Therefore, such copolymers could not be used to make shaped articles